In a wireless communication system, a receiver transmits acknowledgement (ACK) and negative acknowledgement (NAK) signals to a transmitter when received data is successfully and unsuccessfully demodulated, respectively. An ACK/NAK signal requires one bit per codeword.
An ACK/NAK signal needs to be simultaneously transmitted by a plurality of users by using predetermined time and frequency resources. Such multiplexing techniques are classified into frequency division multiplexing (FDM) and code division multiplexing (CDM). FDM is a form of multiplexing where a plurality of different terminals use different time/frequency resources, whereas CDM is a form of multiplexing where a plurality of different terminals use the same time/frequency resources but transmit results obtained by multiplying signals by specific orthogonal codes so that a receiver can identify the plurality of different terminals.
In an uplink, a Zadoff-Chu sequence having an ideal peak to average power ratio (PAPR) is often used. Such a Zadoff-Chu sequence can achieve orthogonality between terminals through a cyclic delay, instead of multiplying a signal by a specific code in the frequency domain.
An uplink ACK/NAK signal is required for a terminal to inform a base station about a successful or unsuccessful (ACK or NAK) receipt of downlink data, and is used to transmit the downlink data.
FIG. 1 illustrates time/frequency resources used by a terminal to perform uplink ACK/NAK signaling in a 3rd generation partnership projection long term evolution (3GPP LTE) system.
Referring to FIG. 1, resources used by one control channel are grouped into two separate resource blocks. Each of the two resource blocks includes N subcarriers along a frequency domain, and 7 orthogonal frequency division multiplexing (OFDM) symbols, which correspond to one slot, in a time domain. One slot has a time duration of 0.5 ms.
In FIG. 1, a plurality of terminals may commonly use one control channel. That is, a control channel A or a control channel B may be shared by the plurality of terminals. In this case, in order to identify the plurality of terminals using the same control channel, a specific code sequence is allocated to each of the plurality of terminals. That is, each of the plurality of terminals generates and transmits a signal spread along a frequency domain and a time domain by using its allocated specific code.
FIG. 2 illustrates a code sequence and a symbol transmitted to each of N subcarriers in an ACK/NAK channel occupying a resource block that includes the N subcarriers in a frequency domain and 7 OFDM symbols in a time domain. In FIG. 2, the resource block corresponding to one slot described with reference to FIG. 1 occupies N subcarriers in the frequency domain and includes 7 symbol blocks BL #0 through #6 in the time domain.
When CDM is used to identify signals of a plurality of terminals, a sequence and a symbol may be mapped to each time/frequency resource as illustrated in FIG. 2. In order to identify the signals of the plurality of terminals, a sequence is applied to each of the frequency domain and the time domain. In FIG. 2, a reference signal is used for channel estimation, and a pre-determined signal between a terminal and a base station is transmitted.
The base station estimates a channel by using the reference signal, and uses a result of the channel estimation so as to demodulate an ACK/NAK symbol transmitted by a control signal. Each time/frequency resource carries a signal multiplied by two or three symbols.
That is, a time/frequency resource on which the reference signal is carried, is obtained by multiplying a frequency domain sequence symbol Cqm(k) by a time domain sequence symbol Ri (i=0, 1, 2). A time/frequency resource on which the control signal is carried, is obtained by multiplying a frequency domain sequence symbol Cqm(k), a time domain sequence symbol Ci (i=0, 1, 2, 3), and an ACK/NAK symbol Q.
In FIG. 2, the frequency domain sequence symbol Cqm(k) indicates a Zadoff-Chu sequence where NZC is the length of the Zadoff-Chu sequence applied to a kth subcarrier in a frequency domain, m is a primary index, and q is a cyclic delay index, and is given by Equation 1.
                                                        C              q              m                        ⁡                          (              k              )                                =                      exp            ⁡                          [                              ⅈ                ⁢                                                                                                                    ⁢                                          2                      ⁢                      π                                                                            N                    ZC                                                  ⁢                                  m                  ⁡                                      (                                                                                            (                                                      k                            -                            q                                                    )                                                ⁢                                                  (                                                      k                            -                            q                            +                            1                                                    )                                                                    2                                        )                                                              ]                                      ,                  k          =          0                ,        1        ,        2        ,        …        ⁢                                  ,                  N          -          1                                    (        1        )            
One sequence is applied to each of the reference signal and the control signal in a time domain. That is, a sequence applied to the control signal in FIG. 2 is expressed as C0, C1, C2, and C3. A sequence applied to the reference signal is expressed as R0, R1, and R2.
Currently, 3GPP LTE considers a configuration in which three reference signals per slot are used for an uplink ACK/NAK channel.
Also, in order to identify a plurality of terminals, a Zadoff-Chu sequence is used along a frequency domain, and a discrete Fourier transformation (DFT) vector, a Walsh-Hadamard sequence, or a Zadoff-Chu sequence may be used in a time domain.
A time domain sequence used for a time domain CDM uses sequences orthogonal to each other. When the number of continuous OFDM symbols along a time domain is Nt, Nt sequences that are orthogonal to each other can be created, wherein the length of the sequences is Nt. When an ith sequence is defined as a row vector Gi=[Ci,0, Ci,1, . . . , Ci, Nt-1], the orthogonality of the sequences to each other is defined as follows.
                    G        i            ·              G        j        +              =                            [                                    C                              I                ,                0                                      ,                          C                              I                ,                1                                      ,                          …              ⁢                                                          ⁢                              C                                  I                  ,                                                            N                      t                                        -                    1                                                                                ]                ·                  [                                                                      C                                      j                    ,                    0                                    *                                                                                                      C                                      j                    ,                    1                                    *                                                                                    ⋮                                                                                      C                                      j                    ,                                                                  N                        t                                            -                      1                                                        *                                                              ]                    =                                    ∑                          k              =              0                                                      N                t                            -              1                                ⁢                                    C                              I                ,                k                                      ⁢                          C                              j                ,                k                            *                                      =                              N            t                    ⁢                      δ                          i              ,              j                                                      where      ⁢                          ⁢              δ                  i          ,          j                      =          {                                    1                                                              if                ⁢                                                                  ⁢                i                            =              j                                                            0                                                              if                ⁢                                                                  ⁢                i                            ≠              j                                          
Theoretically, since the number of resources in the frequency domain is M, and each resource includes three reference signals, as illustrated in FIGS. 3, 4 and 5, M×3 reference signals can be identified by CDM.
Also in the case of a control signal, since the number of resources in the frequency domain is M, and each resource includes four control signals, as illustrated in FIGS. 3, 4 and 5, M×4 control signals can be identified by CDM. However, since each terminal needs to transmit at least one reference signal so that a base station can demodulate a control signal by using the reference signal, the number of terminals that can be identified is M×3. In this case, an orthogonal sequence with a spreading (SF) of 3 is used for the reference signal, and an orthogonal sequence with an SF of 4 is used for the control signal.